This invention relates generally to converter systems, and more particularly to a device and method for premagnetization of a power transformer in a converter system.
Converter systems are often used to connect remote power plants, e.g. wind turbines and photovoltaic (PV) systems, with an electrical supply or distribution grid, e.g. a medium-voltage (MV) grid, or to supply drive devices, e.g. motors, pumps, compressors, etc., from the MV grid. Nowadays, self-commutated voltage-controlled converters (so-called voltage source converters, VSC) with DC voltage intermediate circuits are generally used for these applications. These converters are electrically connected to the MV grid via a power transformer, also referred to as a main transformer or converter transformer, which converts the voltage of the MV grid, usually 1-50 kV, typically 10-30 kV, into a suitable alternating voltage (AC voltage) which, on the AC side of the converter, depending on the application, can be between e.g. 400 V and 3,300 V. A circuit-breaker is arranged on the primary side (grid side) and/or the secondary side (converter side) of the power transformer to connect the converter system as necessary to the grid or isolate it from the grid galvanically.
In applications for renewable energy systems, the main circuit-breaker of the converter system is preferably arranged on the grid side of the transformer. This makes it possible to isolate the power transformer from the grid when it is not in use, e.g. at night time in PV converter systems or in windless conditions in wind turbines, which can result in significant loss and cost savings.
In order to connect the converter to the grid, i.e. to close the circuit-breaker, it is necessary to precharge the DC voltage intermediate circuit of the converter. Otherwise, the discharged DC voltage intermediate circuit would act as a transient short-circuit and high transient short-circuit currents would flow through the power transformer, converter free-wheeling diodes and DC voltage intermediate circuit capacitors. The DC voltage intermediate circuit capacitors would be charged with a large voltage spike, which could destroy these and other converter components. Therefore, a precharging device for converters is generally used in order to precharge the DC intermediate circuit approximately to its rated operating voltage before the circuit-breaker is closed.
If the circuit-breaker is positioned between the grid and the power transformer and the power transformer is not magnetized, closing the circuit-breaker could result in a greatly increased inrush current or inrush currents, in the event of an adverse phase relationship. The magnitude of this inrush current depends on the turn-on time in relation to the timing of the mains voltage and the magnetic flux stored in the transformer core. The strength of the inrush current can, in individual cases, trip fuses or open isolators or circuit-breakers. The power system would have to be sized to withstand the transient inrush currents. However, a repetitive high transformer inrush current can reduce the service life of the circuit-breaker. For this reason, precharging devices for converters are often extended with the function of a premagnetization of the power transformer.
From practice, different devices for precharging the DC voltage intermediate circuit of a converter and for premagnetization of a power transformer are known. Generally, in remote converter systems, no auxiliary power supply by a separate (low-voltage) grid is provided. The entire power requirement for the control and auxiliary equipment, e.g. cooling, heating and lighting subsystems, must then be obtained from the MV grid. This also applies e.g. to PV converters, which can be used at times when no solar power is available for power supply conditioning, e.g. reactive power compensation, as well as for the auxiliary power supply required for the purpose of heating, monitoring, etc., independent of the availability of the solar power. Therefore, an auxiliary power supply is generally provided via an auxiliary transformer from the MV power supply. This auxiliary power supply can also be used to precharge the DC voltage intermediate circuit of a converter.
For example, in a known device, a special three-phase precharging transformer is connected to an auxiliary power supply, the secondary winding of which supplies an uncontrolled three-phase rectifier bridge. The DC side of the rectifier bridge is connected to the DC intermediate circuit of the converter. The rated power of the precharging transformer is selected to be very low, so that on switching on, the transformer impedance limits the precharging current for the DC voltage intermediate circuit, which allows a slow precharging of the DC voltage intermediate circuit capacitor. It is also known to route the connection between the precharging transformer and the DC voltage intermediate circuit via two or more parallel paths of different resistances to controllably limit the precharging current for the precharging procedure.
Once the DC voltage intermediate circuit has been precharged to approximately the rated operating voltage, the converter can be operated by actuating its power semiconductor switches to generate an AC voltage on its AC side, so that the three-phase primary voltage on the grid side of the power transformer, in terms of amplitude, phase and phase sequence to the three-phase grid voltage, is largely synchronized before the circuit-breaker is closed. The respective reference quantities of the voltage amplitude, phase angle and rotation direction of the MV grid are generally measured directly on the three-phase MV grid voltage.
Such a precharge and premagnetization device is relatively complex and expensive in terms of the circuit implementation, the components required, the measurement of the parameters, and the control of the device. There is always a desire to reduce the efforts and costs.
U.S. Pat. No. 9,337,762 B1 describes a premagnetization transformer for a power transformer having a three-phase auxiliary transformer whose primary windings are connected to the three-phase AC grid voltage, and whose secondary windings are coupled to the secondary windings of the power transformer to premagnetize the latter such that a flux and a voltage of its primary windings are in phase with the grid AC voltage. This solution is complicated and costly, in part because of the use of the three-phase auxiliary transformer.
US 2016/0126858 A1 describes a precharging and premagnetization device for a converter system that can be connected to an MV grid, wherein the precharging occurs by means of a separate DC precharging power source. Once the DC voltage intermediate circuit has been precharged to the rated voltage, the converter is suitably controlled in order to modulate, on its AC side, an AC voltage synchronous with the amplitude and phase of the grid voltage. As a result, inrush currents that flow through the chokes and capacitors of an input filter of the converter can be limited when the converter is switched onto the MV grid. The reference quantities for the amplitude and phase of the grid voltage required for the premagnetization are measured directly on the grid voltage side. The effort is relatively high.
U.S. Pat. No. 7,092,262 B2 describes a precharging circuit for a DC intermediate circuit of a three-phase converter connected to a three-phase grid via a power transformer. The precharging circuit uses a single-phase isolating transformer whose input terminals are connected to a single phase of the AC grid voltage and a full-wave rectifier that connects the output of the transformer to the DC voltage intermediate circuit. A premagnetization of a power transformer is not provided.
Known premagnetization devices require measuring equipment on the MV grid side and/or a three-phase auxiliary transformer to derive the reference quantities to synchronize the voltage when magnetizing the power transformer. This leads to a relatively high level of circuitry-related and procedural effort and relatively high costs for the implementation and operation of such converter systems.